Light Entertainment

Sunday, March 4th, ©2012 Marcus Brooks

My mind takes on a scientific spark,
As it would study light, seeks out the dark.

Calcite (Iceland Spar) (Photo by Furrfu via Wikimedia Commons)

Seeing double through a piece of Iceland Spar (Photo by Furrfu via Wikimedia Commons)

When I wrote that couplet, I was thinking about a girl; as men often do when they write couplets. But the indirect reference was to Sir Isaac Newton, who used to spend time in a darkened room, studying light.

This comes to mind because today I happened to look in a mirror while wearing a pair of “passive” 3D glasses (the grey kind, not colored, and without batteries). If you have a pair, you could put them on and try for yourself: look in a mirror, close one eye, then open that eye and close the other. You can only see the open eye! Whichever lens you look through is transparent grey, the other lens is black.

This happens because light rays can act like waves. “Whole” light waves wiggle up-down, left-right, and every angle in-between. But light can be polarized by blocking every wave angle but one. In passive 3D glasses, the plastic molecules line up in a grid. One lens only lets vertical (up-down) waves through, the other only horizontal (left-right) waves.

When you look in a mirror through 3D glasses, your open eye can only see polarized light if it matches the polarity of the lens you’re looking through. The light that illuminates your open eye passes through the same lens both ways, so of course it’s polarized the same. Your closed eye is illuminated too, but light from that side can’t get through both lenses, so the lens on that side looks black.

Why are the glasses polarized? This is how passive 3D TV works. In real life, your eyes see objects from slightly different angles, so you can estimate how far away objects are. A 3D movie is filmed from two different angles. Your TV displays the angles in alternation, polarizing images meant for one eye one way, and the other, the other. Each eye only sees images polarized the same as the lens on that side. Your brain sorts it all out and tells you you’re seeing a 3D movie.

I haven’t found details about how the TV image is polarized, but I bet it’s done with liquid crystal. Liquid crystal acts like a polarizing lens when a charge is applied to it. LCD TVs already use liquid crystals to form images against a white backlight. I assume passive 3D uses liquid crystals instead to polarize the display of a color LED image.

(Active 3D glasses, on the other hand, use liquid crystal shutter lenses to block each eye as the alternating images are shown. That’s why those glasses need batteries.)

In addition to plastics and liquid crystals, light can also be polarized by certain solid crystals, by reflections, and by the sky itself! For example, polarized sunglasses reduce glare because they are polarized to block reflections from horizontal surfaces.

Polarization was first noticed, but not understood, in a crystal called Iceland spar, a form of Calcite. Iceland spar’s crystals not only polarize light, but they refract light of different polarities at different angles, so you see double when you look through it. Because the sky is polarized, a piece of Iceland spar can be used to find the sun’s location in twilight or overcast. It is likely this is the “sunstone” referred to in Viking lore.

Iceland spar was one of the first clues to the nature of light. Light was once considered pure, even holy. (To imagine otherwise was heresy.) Color was thought to be an adulteration, something added to light by objects that it passed through or reflected from. Rainbows were deemed magical, or divine. Light could be bent (refracted), by passing through water or glass, but Iceland spar seemed to actually “break” light so it went in two directions at once!

It was color that led Newton to his darkened room. Telescopes of the day (all refractors) used simple lenses that also tended to break light, in this case blurring the image into different colors around the edges (“chromatic aberration”). Using a prism, Newton found that light could be broken clearly into a rainbow of colors, exactly the same colors and sequence as a rainbow. What’s more, Newton found that he could use another prism to recombine the “broken” beam of light into a pure white beam, just like the original. In this way Newton correctly deduced that the spirit of color, as it were, existed as part of light itself, not as an adulteration. This is why he used the word spectrum (Latin for ghost) to collectively name the colors of light.

Newton considered the idea that light existed as waves, like sound; with color, like pitch, consisting of various wavelengths: red longest, violet shortest. This is, of course, true. We now know “light” (electromagnetic) waves extend far beyond the visible spectrum, from the long-wave VLF pulses used to signal atomic submarines (and “atomic” clocks); to AM, shortwave, FM, and microwave radio signals; to infrared, visible, and ultraviolet light; and finally the shortest waves, X-rays and cosmic rays.

Newton solved chromatic aberration by using a curved mirror to collect and focus light; to this day, an all-reflecting telescope is called a Newtonian. But he never discovered the true nature of light. He gave up his wave theory because some experiments seemed to prove light behaved instead as “corpuscles” (photons). This, of course, is also true. But that’s another story!

[In addition to the websites linked above, thanks to Isaac Asimov’s essay collection titled The Left Hand of the Electron (Dell, 1974).]

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